1. Field of the Invention
The present invention relates to a defective nozzle detector, and an image forming apparatus and system incorporating the defective nozzle detector.
2. Description of the Related Art
An inkjet recording apparatus is known as an image forming apparatus, such as a printer, facsimile, copier, plotter, and multifunctional apparatus combining the above functions, that employs a droplet discharge recording method using a recording head to discharge ink droplets used for image formation.
In image forming apparatuses employing the ink droplet discharge recording method, an ink droplet is discharged from a recording head onto a sheet of paper being conveyed to form an image. (Hereinafter, “image formation”, “recording”, “print”, and “printing” mean the same thing and are used interchangeably.) In addition, such apparatuses are of two main types: A serial-type image forming apparatus, in which the recording head discharges droplets to form image while moving in a main scanning direction, and a line-type image forming apparatus, in which the recording head discharges droplets while remaining stationary.
In such an image forming apparatus of droplet discharging method, ink is discharged onto the sheet from a nozzle of the recoding head, to perform recording. Then, when the viscosity of the ink increases due to the evaporation of the solvent in ink or ink solidification, deposition of impurities and/or mixing in of air bubbles may result in the defective ink discharge and decrease in the produced image quality.
To overcome the above problem, an image forming apparatus is disclosed in which a liquid discharge status detecting unit to detect a status of the head discharging the droplet is provided. When a nozzle that is incapable of properly discharging the droplet is detected, a nozzle surface of the head is cleaned, thereby returning the recording head to its proper state.
As a conventional droplet discharge status detecting unit, for example, JP-2005-280189-A discloses an electrostatic capacitance-type detector including a pair of electrodes disposed on a substrate with a predetermined interval in between; a sensor to deposit a droplet discharged from a nozzle of the liquid droplet discharge head located between the pair of electrodes; and detection means to detect a temporary fluctuation in electrostatic capacitance when the liquid droplet is deposited between the sensor electrodes and to detect, based on the temporary fluctuations in electrostatic capacitance, whether the droplet has been discharged from the nozzle or not.
However, the aforementioned electrostatic capacitance-type detector has a disadvantage in that the detection amount of the ink droplet is not stable because the impact of the droplet is detected by the changes in the electrostatic capacitance relative to the electrostatic capacitance before the impact of the droplet, which is used as a reference capacitance, and after the impact of the droplet. Thus, if the shape of the impacted ink droplet does not expand stably on the sensor substrate, the reference detection amount is not stable, and the fluctuation in the electrostatic capacitance by a subsequent droplet impact cannot be detected. This problem is compounded when a head with a number of nozzles such as a line head is used, multiplying the time required by the number of nozzles.
In such a case, the detection time can be shortened by disposing the electrodes in pairs (for example, n-pairs) in line and detecting multiple nozzles simultaneously. However, a disadvantage of this arrangement is that an ink droplet that has impacted around the adjacent electrode pair expands spanning the two adjacent electrode pair lowers the detection accuracy.
To overcome the above disadvantage, the present invention aims to provide a defective nozzle detector that can detect the droplet discharge state in a short period of time and with a higher accuracy.